Impaired cerebral circulation has been noted following global ischemia utilizing techniques of carbon infusion. The mechanisms responsible are red cell sludging and possibly other factors affecting flow in the ischemic preparation. We have shown that anatomic changes in capillaries do not prevent the passage of red cells through ischemic brain, contrary to what was previously thought. Since the technique of carbon infusion can detect only areas of completely absent flow, we have designed a series of experiments to study regional cerebral blood flow using C14 antipyrine in animals subjected to brief (10 minutes), and therefore clinically relevant periods of circulatory arrest. In an animal model simulating a critical and frequent clinical situation we therefore intend to determine whether or not circulatory impairment exists in conjunction with the earliest development of neurological injury following circulatory arrest.